In a constant velocity universal joint, which is used to construct a power transmission system for automobiles and various industrial machines, two shafts on a driving side and a driven side are coupled to each other to allow torque transmission therebetween, and rotational torque can be transmitted at a constant velocity even when the two shafts form an operating angle. The constant velocity universal joint is roughly classified into a fixed type constant velocity universal joint that allows only angular displacement, and a plunging type constant velocity universal joint that allows both the angular displacement and axial displacement. In a drive shaft configured to transmit power from an engine of an automobile to a driving wheel, for example, the plunging type constant velocity universal joint is used on a differential side (inboard side), and the fixed type constant velocity universal joint is used on a driving wheel side (outboard side).
Irrespective of the plunging type and the fixed type, the constant velocity universal joint mainly includes an outer joint member including a cup section having track grooves formed in an inner peripheral surface thereof and engageable with torque transmitting elements, and a shaft section that extends from a bottom portion of the cup section in an axial direction. In many cases, the outer joint member is constructed by integrally forming the cup section and the shaft section by subjecting a rod-like solid blank (bar material) to plastic working such as forging and ironing or processing such as cutting work, heat treatment, and grinding.
Incidentally, as the outer joint member, an outer joint member including a long shaft section (long stem) may sometimes be used. In order to equalize lengths of a right intermediate shaft and a left intermediate shaft, the long stem is used for an outer joint member on the inboard side that corresponds to one side of the drive shaft. The long stem is rotatably supported by a rolling bearing. Although varied depending on vehicle types, the length of the long stem section is approximately from 300 mm to 400 mm in general. In the outer joint member, the long shaft section causes difficulty in integrally forming the cup section and the shaft section with high accuracy. Therefore, there has been proposed an outer joint member that is constructed by forming the cup section and the shaft section as separate members and applying electron beam welding (Patent Document 1).
Defects such as blowholes and solidification cracks may occur in the welded portion. Thus, a quality check by an ultrasonic flaw detection method is generally performed. Internal defects can be inspected by the ultrasonic flaw detection method. However, the ultrasonic flaw detection method has a problem in that defects which occur in a range of from about 1 mm to 3 mm directly below a surface being affected by a surface reflection echo cannot be detected. In general, such a non-detectable region is referred to as “dead zone”. In Patent Document 2 and Patent Document 3, there have been proposed methods of irradiating ultrasonic waves obliquely in a circumferential direction with respect to a product having a columnar or cylindrical shape.